The present invention relates to underwater pelletizers and pelletizer cutting systems, and more particularly to underwater pelletizers having improved means for maintaining contact between a cutting assembly and the cutting face of an extrusion die.
Pelletizers have been known and used for a number of years. They are used to process rubber compounds, and molten thermoplastics and other polymers into pellets, which may in turn be used in other processes to manufacture various plastic materials.
In an underwater pelletizer, molten plastic is typically extruded by an extruder through orifices in a die coupled to the extruder, thereby tending to form continuous plastic strands. These strands are cut by a cutting system typically having a plurality of knives oriented against the die""s xe2x80x9ccutting facexe2x80x9d to cut the strands as they emerge from the die. The cutting typically takes place under water in an enclosed water chamber or housing.
Plastic pellets are thereby formed which cool and harden in the water contained within the water chamber. Typically, the water in the water chamber is much cooler than the molten polymer, allowing quick cooling of the polymer and quick solidification of the pellets.
An underwater pelletizer is typically constructed so that a constant stream of water passes over the die""s cutting face, conveying the hardened pellets along to equipment which separates out the pellets from the water and dries them.
It is very important that the pellets formed by such a pelletizing process be uniform in size and shape, and that they be made to a specific geometry. There are, however, a number of difficulties which may be encountered in trying to form such uniform pellets.
One of these difficulties relates to the clogging of extrusion orifices in the extrusion die. This difficulty has already been ameliorated by the present inventors, using a system described in U.S. Pat. No. 6,474,696, which is incorporated herein by reference.
Another one of these difficulties relates to the fact that the cutting knives in a typical underwater pelletizer, which are usually borne on a rotating cutting assembly, can xe2x80x9csmearxe2x80x9d the plastic emerging from the die if their blades are not optimally aligned with the cutting face of the die or if they are not urged against the die face with the optimal amount of force. This optimal amount of force, or xe2x80x9ccontact pressurexe2x80x9d, can vary depending upon a number of factors including the nature of the plastic being extruded, and its extrusion temperature and speed and the temperature of the water in the water chamber.
One reason it is difficult to maintain an optimal contact pressure on the cutting knives against the cutting face of the die is that the blades of the cutting knives wear. The die""s cutting face may also wear. As this happens, a gap can form between the blades of the knives and the die""s cutting face. Even a tiny gap can lead to undesirable smearing of the plastic. This is a well known problem with underwater pelletizers.
To overcome this particular problem, prior art systems have implemented somewhat rudimentary strategies for limiting the effects of the wearing of the knife blades. One strategy is to manually move the cutting assembly towards and against the die face whenever the blades have worn to the point where smearing begins to occur. This requires the pelletizer operator, once it is determined that smearing has begun, to mechanically adjust portions of the cutting system to force the cutting knives more closely against the cutting face of the die. This is done at the operators discretion.
This approach relies solely upon the operator""s feel for the blade-to-die face contact pressure and is accordingly dependant upon the operator""s level of experience and expertise, which is undesirable from the point of view of efficiency. Further, since the optimal contact pressure is not necessarily the same for all materials, the operator has to be well trained and experienced to be familiar with dealing with all possible materials.
Another known system provides spring-loaded cutting assemblies which tend to constantly urge the knife blades of the cutting assembly against the cutting face of the die. Springs having different spring rates are often provided to allow use of the system with different materials. However, this type of system does not allow for xe2x80x9cfine-tuningxe2x80x9d of the amount of contact pressure between the blades and the cutting face, which is very important.
For example, if a particular spring does not provide quite enough force, then the extrusion pressure can force the blades away from the cutting face, leading to smearing. If the spring provides too much force, then the blades wear too quickly. Further, a particular spring provides a force dependant upon how much it is compressed or stretched. As the cutting assembly is urged closer to the cutting face as the knife blades wear, the force provided by the spring changes, slightly, but potentially significantly. Accordingly, a spring, and even a set of different springs, cannot provide the fine tuning necessary for all extruded materials.
As noted earlier, xe2x80x9csmearingxe2x80x9d can also occur when the knife blades of the cutting assembly do not conform precisely to the plane of the cutting face of the extrusion die. This is typically caused by the misalignment of the shaft which rotates the cutting assembly. While self-aligning cutting assemblies, such as that shown in U.S. Pat. No. 5,624,688, which issued in 1997 to Adams et al., have been used in previous underwater pelletizers, they typically utilize a complicated and expensive ball-bearing system for maintaining the knife blades coplanar with the cutting face of the die.
An improved pelletizer continues to be needed, therefore, which allows for a more finely-tuned adjustment of the contact pressure between the knife blades and the cutting face of a die in an underwater pelletizer, and which also accomplishes, simply and inexpensively, the maintenance of a parallel, preferably co-planar relationship between the knife blade edges and the cutting face of the extrusion die.
The present invention is an underwater pelletizing system and more particularly an underwater pelletizing system having an improved a cutting system. In a preferred embodiment of the invention, the cutting system has a rotatable cutting assembly bearing a plurality of cutting knives; drive shaft attached to the cutting assembly for rotating it, the drive shaft driven by a drive motor; motorized cutting assembly adjustment means for adjustably moving the drive shaft axially toward and away from the cutting face of the die, thereby moving with it the cutting assembly along its rotational axis; a load measuring system associated with the motorized cutting assembly adjustment means to measure the contact pressure between the cutting assembly and the cutting face of the die; and a control system for coordinating communication between the load measuring system and the motorized cutting assembly adjustment means and for providing instructions to the motorized cutting assembly adjustment means to move the drive shaft toward and away from the cutting face of the die.
In a preferred embodiment of the invention, the system has a frame, and the drive motor is carried on a carriage mounted on and movable relative to the frame. The motorized cutting assembly adjustment means comprises a servomotor for reciprocating the carriage linearly relative to the frame. In one embodiment, the servomotor turns a ball screw having a shaft attached to the carriage for moving the carriage. The carriage may include a number of ball bearings which journal one or more shafts forming a portion of said frame.
The control system is preferably a programmable logic control system which allows pre-programmed cutting programs to be run.
One version of a useful cutting assembly has a center hub fashioned to be threaded onto an end of the drive shaft, and a blade-holding portion. An elastomeric disc is attached to both the center hub and the blade holder, bridging an annular gap between them.